This invention relates to semiconductor integrated circuit devices in general and more particularly to an improved EPROM (Electrically Programmable Read Only Memory) device, and a method of manufacturing the same.
In general, an EPROM device is constructed of a memory array portion and a peripheral circuit portion, such as input/output circuitry and X-Y decoder circuitry, which are formed on the major surface of a single semiconductor substrate, the memory array portion comprising a plurality of MIS (Metal-Insulator-Semiconductor) type memory transistors each having a control gate electrode over a floating gate electrode, the peripheral circuit portion being formed around the memory array portion and comprising a plurality of MIS type transistors.
An EPROM of the nature described above requires the so-called scaling-down in which, in order to render the integration density high and the operating speed high, the channel length (gate length) is made short and the thickness of a gate oxide film is made small. Since, however, writing into a memory requires a voltage higher than that for the reading, there are some parts, especially in the peripheral circuit portion, where the scaling-down is impossible.
In, for example, an n-channel type EPROM, the writing voltage has typically been 25 V and the reading voltage 5 V. Accordingly, all the MIS type transistors of the peripheral circuit portion have been put into a structure capable of enduring the writing voltage, in consideration of the simplification of a manufacturing process. That is, the gate oxide films of the respective MIS type transistors have had comparatively large thicknesses of approximately 1,000 .ANG.. This has made it difficult to realize high speed reading.